- For the first time in medical history scientists have corrected the cause of Huntington’s disease (HD)
- HD is a fatal congenital neurodegenerative disorder that causes uncontrolled movements, emotional challenges, and loss of cognition
- Current treatments only help symptoms rather than slow the progression of the disease
- Researchers from University College London (UCL)havesafely lowered the levels of toxic proteins in the brain that cause HD
- Experts say this is the biggest breakthrough in neurodegenerative research for 50 years
- Earlier, an American animal study successfully used a similar technique to “silence” the mutant huntingtin gene in mice brains
- Gene silencing stops the gene from making any mutant protein but does not eradicate the mutant HD gene
- More studies are necessary to show whether the UCL study will effectively change the course of HD
- Gene editing is a game-changer in biomedical research, but it faces significant technical and ethical challenges
Huntington’s disease and gene silencing
In December 2017, scientists completed the first human genetic engineering study that targeted the cause of Huntington’s disease (HD) (also known as Huntington’s Chorea), and successfully lowered the level of the harmful huntingtin protein that irreversibly damages the brains of patients suffering from this incurable degenerative condition. Current treatments for HD only help with symptoms, rather than slow the disease’s progression. The study’s leader, Professor Sarah Tabrizi, director of the Huntington’s Disease Centre at University College (UCL) London’s Institute of Neurology, says, “The results of this trial are of ground-breaking importance for Huntington’s disease patients and families”. Tabrizi’s research followed an earlier American study, which successfully used a similar technique to “silence” the mutant huntingtin gene in mice brains.
This Commentary describes Huntington’s disease, the 2 studies to silence the huntingtin gene, and also the gene silencing technology, which underlies both studies.
Huntington's disease
Huntington’s disease (HD) is a fatal congenital neurodegenerative disorder caused by a mutation in the gene of a protein called huntingtin, which triggers the degeneration of cells in the motor control regions of the brain, as well as other areas. HD is one of the most devastating neurodegenerative diseases, which some patients describe as Parkinson’s, Alzheimer’s and Motor Neurone disease rolled into one. HD leads to loss of muscle co-ordination; behavioural abnormalities and cognitive decline. Generally if one parent has HD then each child has a 50% chance of inheriting the disease. HD affects both sexes and about 12 people in 100,000, but appears to be less common in people of Japanese, Chinese, and African descent. If a child does not inherit the huntingtin gene, s/he will not develop the disease and generally cannot pass it to subsequent generations. Although there is a wide variation in its onset age, the majority of HD patients are diagnosed in middle age. Currently there is no cure for the disorder: although drugs exist, which help manage some symptoms they do not influence the progression of the disease.
Signs and symptoms
The characteristic symptoms of HD include, cognitive impairment, mood shifts, irritability, depression and behavioural changes. As the disease develops symptoms get progressively worse and include uncontrolled movements, cognitive difficulties and issues with speech and swallowing. HD typically begins between the ages of 30 and 50. An earlier onset form called juvenile HD occurs in people under 20. Symptoms of juvenile HD differ somewhat from adult onset HD and include unsteadiness, rigidity, difficulties at school, and seizures.
Diagnoses
A genetic test, together with a medical history and neurological and laboratory tests, support doctors in their diagnosis of HD. Genetic testing, which costs between US$250 and US$350, is both cost-effective and diagnostically precise, and is important to establish whether HD is present in a family because some other illnesses may be misdiagnosed as HD. The disorder is a model for genetic testing because HD is relatively common, its etiology is understood, and there is significant experience with its management. There are 3 main types ofHD genetic testing: (i) to confirm or rule out the disorder, (ii) pre-symptomatic testing, and (iii) prenatal testing. Persons at risk of HD often seek pre-symptomatic testing to assist in making decisions about marriage, having children, and career. Positive results can evoke significant adverse emotional reactions, so appropriate pre- and post-test counselling is important.
Treatment
Current treatments can only alleviate the symptoms of HD, and do not delay the onset or slow the progression of the disease. Until the findings of the Tabrizi study there was no treatment that could stop or reverse the course of the disorder. Tetrabenazine and deuterabenazine are drugs prescribed for treating the chorea associated with HD. Antipsychotic drugs may also help to alleviate chorea and can be used to help control hallucinations, delusions, and violent outbursts associated with the disease. Drugs may be prescribed to treat depression and anxiety, which are relatively common among HD sufferers. Drugs used to treat HD may have side effects such as fatigue, sedation, decreased concentration, restlessness, or hyper-excitability, and only should be used when symptoms create problems for the individual.
The Emory Study
In June 2017 scientists from the Emory University School of Medicine in Atlanta, USA, published findings of an animal study in the Journal of Clinical Investigation, which used the gene editing technique CRISPR-Cas9 to “silence” the mutate huntingtin gene (mHTT) in mice brains.
Study leader Xiao-Jiang Li, professor and expert in molecular mechanisms of inherited neuro-degeneration, used adult mice engineered to have the same mutant Huntington's-causing gene as humans, and were already showing signs of the disease. Using CRISPR-Cas9 Xiao-Jiang introduced genetic changes in an afflicted region of the brain that prevented further production of the faulty huntingtin gene. After 3 weeks, researchers noted that the brain region where the vector was applied, the mice brains showed that the aggregated proteins had almost disappeared, and there was a concomitant improvement in their physical functions; although not to the levels of the control mice.
The Emory research team’s findings showed that CRISPR-Cas9 successfully silenced part of a gene that produces toxic protein aggregates in the brains of mice, and demonstrated that the technique holds out the possibility of a one-time solution for HD.
The UCL study
What the Emory study achieved in mice the UCL study achieved in humans. The UCL study of the huntingtin-lowering drug Ionis-HTTRx led by Tabrizi and sponsored by Ionis Pharmaceuticals, a US$6bn NASDAQ traded company based in Carlsbad, California, used a similar technique as the Emory study to “silence” the mutated huntingtin gene. The study, which had been in pre-clinical development for over a decade, enrolled 46 human patients with early HD in 9 study centers in the UK, Germany and Canada. Each patient received 4 doses of either Ionis-HTTRx or a placebo, which were given one month apart by injection into the spinal fluid to enable the drug to reach the brain. As the study progressed, the dose of Ionis-HTTRx was increased several times according to the ascending-dose study design.
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